Abstract Introduction Study Area Aquifer Framework and Definitions Transmissivity, Hydraulic Conductivity and Storage Coefficient Hydrogeology - Sediment Hydraulic Conductivity   >  W. Broward Co.   -  Central and E. Broward Co. - Aquifer Delineation - Transmissivity Distribution Ground-Water Flow System Summary References Cited PDF Version

A generalized west Broward County hydrogeologic section begins at land surface with a few feet of peat, muck, and lime mud of freshwater limestone. (At the drill sites, some of these materials have been replaced with road or levee fill.) Below, to depths of 30 to 40 feet (fig. 15, fig. 17, fig. 18, and fig. 20) are interbedded materials of the upper part of the Fort Thompson Formation consisting of hard, dense, cream, gray, or brown limestone, particularly in the upper part (fig. 23); sand or sand and shell with lime mud matrix; and some limestone with poorly to moderately developed solution-cavity zones. These upper beds generally retard vertical movement of water, but thin zones with cavities may be very highly permeable.

Below the upper part of the Fort Thompson Formation is the very highly permeable zone of the Fort Thompson Formation (fig. 24) in the Biscayne aquifer. Along Alligator Alley (fig. 3), this zone extends from about 30 to 50 feet below land surface (fig. 17). At the Alligator Alley west site (G-2330, fig. 17), an extremely productive cavernous zone was found at 39 to 40 feet in four wells. In contrast, 6 miles east (G-2320, fig. 17), the 30- to 50-foot interval contains abundant sand, shell, and lime mud that is interbedded with, or fills cavities in, the limestone, greatly reducing the hydraulic conductivity. This suggests that there may be considerable variability of hydraulic conductivity and transmissivity in the Biscayne aquifer in west Broward County (see table 4 for test results).

Along the eastern edge of Broward County and southward from Alligator Alley, the very highly permeable zone thickens and the base deepens to about 70 feet (G-2311 and G-2317, fig. 20). Locally, highly permeable limestone beds of the Tamiami Formation that immediately underlie the Fort Thompson Formation are included in the Biscayne aquifer. Near the western edge of the county (for example, G-2329 and G-2340, fig. 17 and fig. 19), the permeable limestone is commonly replaced by 40 to 70 feet of mixed lime mud, sand, and shell, or slightly indurated limestone having low permeability, except at the southwest Everglades site (fig. 19, G-2338) where about 35 feet of highly permeable shelly limestone occurs (G-2338B, table 4). A slug test in the lime-mud- shell-sand mixture at the Alligator Alley Snake Road site (G-2329B, table 5) indicated a hydraulic conductivity of 0.16 ft/d. North of Alligator Alley the Fort Thompson Formation thins, and at the North Everglades west, central, and east sites (fig. 15, G-2314, G-2313, and G-2315, respectively), only minor very permeable zones occur in beds that are primarily dense limestone or muddy sand. Thus, the Biscayne aquifer does not extend to those sites.

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Figure 23. (left) Hard, dense limestone of low permeability of the Fort Thompson Formation that occurs at shallow depths in west Broward County. (a) very tight caprock at site G-2315; and (b) representative sample of low-permeability limestone at site G-2330 (0 to 30 feet deep). [click on images above for larger version]	Figure 24. (right) Very highly permeable limestone of the Fort Thompson Formation in south-central and west Broward County. (a) porous limestone with chalky cavity linings at 58 to 60 feet deep at site G-2317; and (b) porous, sandy, shelly limestone blown out of well open at 40 to 65 feet deep at site G-2311. [click on images above for larger version]

From 20 to 60 feet of green, clayey sand and shell and limestone of the upper part of the Tamiami Formation, mostly of low to moderate permeability (G-2329, G-2330, G2320, and G-2319, fig. 17) occur below the Fort Thompson Formation in west Broward County. Green clay or green claystone, sandstone, and lime mud also occur, usually in minor quantities. These materials of the upper part of the Tamiami Formation and the lime mud-sand-shell beds of the Fort Thompson Formation previously described (along the western edge of Broward County) form a semiconfining unit that overlies the gray limestone aquifer and separates it from the Biscayne aquifer wherever the latter occurs in west Broward County (fig 7b). The degree of confinement varies areally. Pumping a 6- inch well in either unit (Biscayne aquifer or gray limestone aquifer) at the Alligator Alley central site (G-2320, fig. 17) quickly causes water-level drawdown on the unpumped unit equal to 15 to 20 percent of that in the pumped unit. This semiconfining unit is also leaky at the north Everglades central site (G-2313, fig. 15). Pumping well G-2313C (see table 4), open to the gray limestone between 106 and 146 feet, at about 400 gal/min for 25 to 30 minutes resulted in measured drawdowns of 7.21 feet (includes calculated well loss of 2.54 feet) in the pumped well, 0.73 foot in well G-2313B (open 46 to 81 feet), and 0.12 foot in well G-2313A (open to the water-table aquifer between 12 and 22 feet). Both observation wells are 15 feet from the pumped well. However, confinement is much greater in westernmost Broward County where the semiconfining unit is primarily green clay, silt, or a lime mud-sand-shell mixture (G-2338, between 50 and 101 feet; G-2346, between 28 and 60 feet; fig. 19) of low to very low permeability. At the southwest Everglades site (G-2338), leakage was not apparent during a 100-minute pumping test of well G-2338C open to the gray limestone. A bailer test of well G-2338H, open to a mixture of soft sand, silt, and clay in the upper part of the semiconfining unit, indicated a hydraulic conductivity of 0.061 ft/d (table 5). The unit becomes increasingly more clayey and stiff with depth at that site, and consequently, is less permeable than the tested interval. At site G-2338, the unit has the lowest permeability of any of the test sites in west Broward County.

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Figure 25. (left) Gray, shelly, highly permeable limestone of the Tamiami Formation in west Broward County. (a) 119 to 123 feet deep, site G-2320; and (b) 130 to 140 feet deep, site G-2319. [click on images above for larger version]	Figure 26. (right) Silty, very fine sand with phosphorite and stiff, green, silty clay of the Tamiami Formation in west Broward County. (a) 239 to 243 feet deep, site G-2315; and (b) 209 to 212 feet deep, site G-2313. [click on images above for larger version]

Gray, shelly, relatively soft limestone (fig. 25) of the Tamiami formation in west Broward County forms a water-bearing unit, referred to as the gray limestone aquifer in this report. The top of this aquifer is usually found at a depth of about 70 to 100 feet below land surface (fig. 17, fig. 18, fig. 19, except in the northwest corner where it occurs at about 35 to 40 feet below land surface (fig. 17, fig. 18, fig. 19), except in the northwest corner where it occurs at about 35 to 40 feet below land surface (G-2313 and G-2314, fig. 15 and fig. 19). Minor quartz sand or tan limestone lenses (probably less permeable than the gray limestone) occur near the base of the aquifer at some sites. Observations made during drilling and examination of the lithologic samples suggest that the aquifer may be divided into two layers in many places–an upper layer that is less permeable and commonly thinner than the lower more permeable and more consolidated layer. The upper layer is much thicker and nearer the surface at sites G-2314 and G-2313 (fig. 15 and fig. 19) where there is a poorly consolidated and cemented facies of the lime mud-shell-sand mixture that occurs farther south (G-2340 and G-2329, fig. 19). At the North Everglades east site (G-2315 between 170 and 185 feet, fig. 15) there is a third deep layer that appears to be in a local basin, separated from the overlying layers by sand and less-permeable limestone. The thickness of the aquifer ranges from about 30 to 110 feet in west Broward County.

Aquifer test (G-2320J, G-2330Z and G-2338C, table 4 indicate the hydraulic conductivity of the gray limestone aquifer is about 900 ft/d in the central and southwestern part of west Broward County. Apparently, there is a decrease in hydraulic conductivity and transmissivity near the eastern edge of west Broward County, on the basis of a hydraulic conductivity of 590 ft/d at the Alligator Alley east sites (G-2319X, table 4; G-2319, fig. 17) and the presence of less-permeable or less-productive (much lower yields during drilling) facies found at sites to the south, east, and north (G-2311 and G-2312, fig. 20; G-2321, fig. 17; G-2312 and G-2341, fig. 16). The test value of 650 ft/d (G-2313C, table 4; G-2313, fig. 15) at the north Everglades central site and the lithologies and lower yields at adjacent sites to the east and west suggest a general decrease in hydraulic conductivity in that area. The apparent decreases to the east and north may correlate with changes from a limestone having abundant coarse shell fragments and shells to a more fine-grained limestone with smaller pore spaces and containing more quartz sand, silt, and clay. The poorly consolidated upper layer that occurs in northwest Broward County probably has a lower hydraulic conductivity than the main part of the aquifer, as indicated by relatively low yields obtained during test drilling and a value of 280 ft/d from a test at the north Everglades central site (G-2313B, table 4; G-2313, fig. 15).

Below the gray limestone, from 0 to 25 feet of limestone of moderate to low permeability (for example, G-2315 and G-2316, fig. 15 and fig. 18) may occur locally above a 20- to 40-foot sequence of fine quartz sand. The quartz sand locally may contain moderate amounts of carbonate sand in the upper part. The sand has relatively little clay or silt in the upper part but becomes greenish and more phosphatic and clayey with depth (fig. 26a) until green clay or silt is found (fig. 26b). At some places, the green clay is very stiff, and at others, it is softer and less compact or cohesive. The transition from muddy sand to clay or silt, with the consequent permeability contrast, marks the base of the surficial aquifer system in west Broward County. The base occurs at a relatively uniform depth of about 160 to 200 feet in that area.